1. Field of the Invention
The present invention relates to a process for preparing a water-soluble organic oxide which is useful as a raw material for various chemicals and, more particularly, to an industrially advantageous process for preparing a water-soluble organic oxide, such as 5-amino-4-oxo-2-pentenoic acid, using inexpensive molecular oxygen as an oxidizer and recycling the sensitizer used in the reaction.
2. Description of the Background Art
Oxidation reactions using singlet oxygen, wherein molecular oxygen is reacted in the presence of a sensitizer under irradiation of light, are important reactions in processes for manufacturing chemical products because of the excellent reactivity and selectivity and abundant availability of the molecular oxygen from atmosphere. Included in these reactions are an O.sub.2 addition reaction using high energy singlet state oxygen molecule produced by energy from a sensitizer which has been excited by irradiation of light, an oxidation reaction involving secondary decomposition of O.sub.2 adducts, and the like. Given as a specific example of the process using such an oxidation reaction is a process for preparing an N-protected 5-amino-4-oxo-2-pentenoic acid, a raw material for the synthesis 5-aminolevulinic acid which is useful as various chemicals, a herbicide, and the like. According to this process, an N-protected 5-amino-4-oxo-2-pentenoic acid can be prepared by reacting furfurylamine, of which the amino group has been protected, with oxygen molecule under irradiation by light in the presence of a sensitizer in pyridine (U.S. Pat. No. 5,380,935).
Because the oxidation reaction with singlet oxygen has been conventionally carried out in a system wherein the raw material and a sensitizer are homogeneously dissolved in a solvent, the reaction product is obtained as a mixture with the sensitizer in the solvent and it was difficult to isolate the reaction product from the mixture. Furthermore, because a sensitizer generally absorbs visible lights, it conspicuously dyes the products if contained in the products even in a trace amount. A step of removing the sensitizer after the reaction was thus required for obtaining a high purity product.
A method of adsorbing the sensitizer in activated carbon and separating out the activated carbon is a most commonly used method for the removal of the sensitizer from the oxidation reaction products. According to this method, not only it takes a long period of time for the sensitizer to be adsorbed in activated carbon, but also it is difficult to completely remove the sensitizer. In addition, because the sensitizer adsorbed in activated carbon and the activated carbon used for the adsorption cannot be regenerated for reuse, it is difficult to continuously carry out the oxidation reaction. This gives rise to a high production cost of the organic oxides.
As a means for overcoming this problem, a method of using the sensitizer carried on a carrier such as an ion-exchange resin has been proposed. The sensitizer carried on a carrier, however, exhibits only low efficiencies in both irradiation by light and oxygen activation. The method was thus not practical.